In a gas turbine engine a stage of stator vanes is disposed between rotor stages, such as in a turbine section having at least two rotating stages. Segmented outer air seals surround the rotating stages and define portions of the gas flow path. It is desirable that the blade tips be spaced as close as possible to the outer air seals to minimize leakage of gases around the tips of the blades. As a result of thermal growth and centrifugal forces the radial location of the blade tips varies during engine operation. It is, therefore, common to support the air seal segments and to control the temperature of the structure supporting the segments in a manner adapted to have them move radially simultaneously with and in the same direction as the blade tips in order to maintain as small a gap therebetween throughout various engine operating modes.
Commonly owned U.S. Pat. No. 3,990,807 shows outer air seals radially supported entirely by structure attached to the radially outer ends of axially adjacent stator vanes. The outer ends of the vanes supporting the seal segments are free to move radially relative to an outer casing. The outer air seal segments thereby move in concert with the outer ends of the stator vanes to which they are attached. Stator vane growth is assumed to match rotor blade growth such that a small clearance is maintained between the blades and the outer air seals. One object of that invention is to provide for support and growth of the outer diameter of the stator vanes independent of a surrounding annular turbine casing.
In commonly owned U.S. Pat. No. 3,992,126, each outer air seal segment of a first rotor stage has its front and rear end radially supported and located from respective front and rear annular support rings attached to a surrounding turbine casing. Outer seal segments for the following rotor stage are similarly secured to another pair of axially spaced apart support rings which are fixed to the turbine casing. The front and rear feet of stator vanes disposed between the adjacent rotor stages are secured to the two centrally located rings of the above mentioned four support rings. Other patents showing segmented outer air seals secured at their forward and rearward ends to respective forward and rearward annular support rings fixed to an outer turbine casing are commonly owned U.S. Pat. Nos. 3,957,391 and 3,975,112.
Commonly owned U.S. Pat. Nos. 4,019,320 and 4,069,662 also show segmented outer air seals for successive turbine stages secured to separate pairs of axially spaced apart rings or flanges which are in turn secured to or integral with a surrounding casing. In those patents the clearance between the outer air seals and the blade tips is controlled by directing cooling air against the casing, whereby the outer air seal segments move radially in concert with the casing. Controlling the growth of the casing thereby controls movement of the air seals. In the embodiment shown therein cooling air is directed from five circumferentially mounted, axially spaced apart spray bars, against four axially spaced apart casing flanges or flange joints for the purpose of controlling the radial movement of only two stages of outer air seals.